Modern mobile equipment, such as smart phone, wearable gadget (eye glasses, wrist watch, etc.), portable computer, tablet computer, hand-held game console, automobile navigation device, digital camera and digital camcorder, etc., is equipped with wireless accessibilities to different remote systems for various services, including mobile telecommunication systems of different generations (e.g., 3G and 4G) for interchanging voice, text messages, e-mails and/or data, global navigation satellite positioning system (GLASS) for locating, and/or local network system (e.g., Wi-Fi network system) for interchanging information.
To correctly and fully access and utilize services involving a remote system, the mobile equipment works as a terminal to synchronize with the remote system by adjusting local clock and/or local signal timing to keep on aligning timing received and/or retrieved from the remote system, e.g., aligning start, middle and/or end of symbol(s), frame(s), sub-frame(s), packet(s), beacon(s), pilot(s), header(s), delimiter(s) and/or synchronization word(s) in a remote signal received from the remote system. After achieving synchronization with a remote system, the terminal obtains a corresponding system time via synchronization with the remote system; for example, the terminal may obtain a value of the system time whenever timing alignment occurs during synchronization with the remote system, wherein value of the system will update (increase or decrease) with time, and therefore can reflect elapse of time. For example, if the remote system is a mobile telecommunication system, the corresponding system time may be valued in terms of SFN (system frame number), and timing alignments to the remote system may be achieved by performing frame synchronizations. If the remote system is GNSS, the corresponding system time may be valued in terms of TOW (time of week), and timing alignments to the remote system may be achieved by performing PVT (position, velocity, time) fixings. If the remote system is Wi-Fi network system, the corresponding system time may be valued in terms of symbol timing, and timing alignments to the remote system may be achieved by performing symbol synchronizations. When the synchronization with the remote system ends or is lost, e.g., when the terminal stops receiving remote signals from the remote system, the terminal cannot track and maintain the corresponding system time, since there is no remote signal for the terminal to align.